This invention relates to methods of producing semiconductor devices. More particularly, it is concerned with methods of depositing silicon nitride films on substrates in the manufacturing of semiconductor devices.
Silicon nitride films are widely used in the manufacture of semiconductor devices and integrated circuits. Plasma deposition of silicon nitride is a relatively low temperature process which is used frequently so as not to cause harmful effects to previously fabricated device structures. Such a process is particularly desirable for the deposition of final passivating films for protecting the device surfaces.
In the past silicon nitride films have been plasma deposited by employing a gaseous mixture of silane and ammonia together with a suitable carrier gas such as argon, helium, or nitrogen and using a high frequency RF plasma. High frequency RF plasma techniques frequently produce a deposited film which is under tensile stress causing cracks in the film upon subsequent high temperature manufacturing operations. Films deposited by this method may also contain impurities resulting from the difficulty in purifying the ammonia reactant. In addition ammonia/silane nitride films have a considerable amount of hydrogen incorporated in the deposited film. When these films are an integral part of the active device structure, the incorporated hydrogen has been shown to be detrimental to device stability. When these films are applied as the final protective passivation layers on MOS devices, the hydrogen atoms can diffuse into the gate oxide and create traps for hot electrons.
Because of these problems, there have been efforts to reduce the amount of hydrogen in plasma deposited silicon nitride films. Nitrogen has been substituted for ammonia as the nitrogen source during the deposition step. In other efforts to reduce the concentration of hydrogen, fluorine in the form of SiF.sub.4, NF.sub.3, or F.sub.2 has been employed as a reactant in addition to the silane during the deposition process. The purpose of the fluorine is to replace relatively weak Si-H bonds with stronger Si-F bonds to improve the deposited silicon nitride film. In addition, efforts have been made to change the stress in the deposited film from tensile to compressive by using low frequency, typically below 1MHz, rather than high frequency RF for generating the plasma.